Add more validation for free-form server fields (#272)

<!-- Provide a brief summary of your changes -->
The changes are intended to create validators for all the free-form
fields. Please find the details of the validation of different field
below.

- Repository.Source: only github and gitlab is accepted.
- Repository.Url - It should be a valid github or gitlab URL. Created
regex for both.
- Server.Name - Max 200 char can be there. Reverse domain name
verification is part of another PR (not owned by me).
- Package.Name - Max 200 char and no spaces.
- Server.Description - Max 100 char.
- Remote.Url - It should be valid URL.

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
The motivation was to make the code clean and add base code for more
validator for different objects like Repository etc.
https://github.com/modelcontextprotocol/registry/issues/97

## How Has This Been Tested?
It's tested via UTs. Added UT's for all the validations.

## Breaking Changes
Yes

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [x] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [ ] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

Co-authored-by: Avish Porwal <avishporwal@microsoft.com>
Co-authored-by: Adam Jones <adamj+git@anthropic.com>

internal/validators/constants.go

@@ -0,0 +1,23 @@
+package validators
+
+import "errors"
+
+// Error messages for validation
+var (
+	// Repository validation errors
+	ErrInvalidRepositoryURL = errors.New("invalid repository URL")
+
+	// Package validation errors
+	ErrPackageNameHasSpaces = errors.New("package name cannot contain spaces")
+
+	// Remote validation errors
+	ErrInvalidRemoteURL = errors.New("invalid remote URL")
+)
+
+// RepositorySource represents valid repository sources
+type RepositorySource string
+
+const (
+	SourceGitHub RepositorySource = "github"
+	SourceGitLab RepositorySource = "gitlab"
+)